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Jae-Hwang Lee, the head of the Nano-Engineering Laboratory in the UMass Amherst Mechanical and Industrial Engineering (MIE) Department, is a member of a multi-institutional and multi-disciplinary team of researchers who co-authored a fundamental materials research article on cold spray additive manufacturing published in the prestigious journal Nature Scientific Report. Cold spray is a materials consolidation process that utilizes micron-sized particles and accelerates them at supersonic velocities through a de Laval rocket nozzle. The impacting particles undergo extreme plastic deformation and then consolidate, thus forming a dense coating with a near net-shaped quality.

Cold spray can create metal parts with properties comparable to those of wrought metals for certain alloys, such as 5056 Aluminum. Cold spray can also be used to join dissimilar metals and to repair damaged parts. The process can be used for metals, polymers, cermets, and/or combinations of these to produce materials not achievable by conventional technologies.

“Since cold spray is based on microscopic supersonic collisions of feedstock powders, understanding material’s extreme behavior is critical,” says Professor Lee about the significance of the article, which is titled “Dynamics and extreme plasticity of metallic microparticles in supersonic collisions.”

Lee adds that the United States Army Research Laboratory, which participated in the article’s research and authorship, is supporting various UMass Amherst faculty members to study cold spray additive manufacturing through collaborative research and separately funded efforts.

The authors of the Nature Scientific Report article are: Wanting Xie of the UMass MIE and physics departments; Arash Alizadeh-Dehkharghani, Qiyong Chen, and Sinan Müftü of the MIE department at Northeastern University; Victor K. Champagne, Director of the ARL Cold Spray Center at the United States Army Research Laboratory, Aberdeen Proving Ground, in Maryland; Xuemei Wang and Aaron T. Nardi of the United Technologies Research Center, East Hartford, Connecticut; Steven Kooi of the Institute for Soldier Nanotechnologies at the Massachusetts Institute of Technology; and Lee.

Despite the growing interest in cold spray as an additive manufacturing method, the materials science underlying the extreme microscopic events in general has not been explicitly understood, primarily because the material response is in a nonlinear, non-equilibrium, and high strain-rate regime. Such fundamental research into cold spray materials science is the purpose of the research described in the Nature Scientific Report article.

“The study of single-particle impact has led to a better understanding of the bonding mechanism associated with cold spray,” states Champagne.

Lee and his colleagues explain that high-velocity impacts of microparticles often occur in either natural or artificial environments. While sand, ice particles, and minute space debris produce destructive effects on wind turbines, aircraft, and spacecraft, in certain manufacturing methods such as shot peening, sand blasting, and fluid-jet polishing, speeding microparticles create beneficial properties on the impacted surface of substrate by virtue of impact-induced extreme conditions. Although the impacting microparticles are generally discarded after engineering processes, in this emerging additive manufacturing method called cold spray, supersonically accelerated metal microparticles are consolidated through extreme plastic deformation.

The material characteristics of particles attained from these extreme collision events essentially govern the performance of the final macroscopic object. Moreover, as this solid-state consolidation process is typically completed in a very short temporal scale, even nanoscale morphologies of metals can be preserved without significant recrystallization. This aspect is particularly advantageous when nanoscale grains with a large gradient are needed.

Champagne also indicates that “Theoretical models have been validated and refined through the empirical data provided by the UMass micro-ballistic study.”

According to the article published in Nature Scientific Report, metallic microparticles can acquire remarkable nanoscale morphologies after experiencing these high velocity collisions, but materials science regarding the extreme events has been limited due to a lack of controlled experiments.

“In this work,” the article explains, “collision dynamics and nonlinear material characteristics of aluminum microparticles are investigated through precise single-particle collisions with two distinctive substrates, sapphire and aluminum, across a broad range of collision velocities, from 50 to 1,100 m/s.”

Thereafter, an empirical constitutive model can be calibrated based on the experimental results and is used to investigate the mechanics of particle-deformation history. Real-time and post-impact characterizations, as well as model-based simulations, show that significant material flow occurs during the impact, especially with the sapphire substrate.

“The presented methodology, based on the use of controlled single-particle impact data and constitutive models, provides an innovative approach for the prediction of extreme material behavior,” as the article says. “Therefore, materials engineering using the localized extreme mechanical events can allow us an unconventional metallurgical method to precisely control nanoscale morphologies of bulk metals.”

Dr. Lee has been an assistant professor in the UMass MIE department since 2014. He received a Ph.D. degree from Iowa State University in Condensed Matter Physics for 3D photonic crystals and their energy applications. For his postdoctoral study, Dr. Lee worked at the Institute for Soldier Nanotechnologies at MIT, and then he worked as a research scientist in the Richard Smalley Institute at Rice University until 2014. (July 2017)